1. Field of the Invention
The present invention relates to a method for fabricating a structure of etch-resistant metal/semiconductor compound using resistless electron beam lithography, more specifically a highly focused electron beam to produce a sub-micron structure of etch-resistant metal/semiconductor compound.
2. Brief Description of the Prior Art
The fabrication of ultra-small scale electronic devices requires efficient high resolution lithography techniques. Resist-based lithography processes are very frequently involved in these high resolution lithography techniques, and poly(methyl methacrylate) (PMMA) is the polymer most widely used as a resist for electron beam lithography applications (S. P. Beaumont, P. G. Bower, T. Tamamura, C. D. W. Wilkinson, Appl. Phys. Lett., 38, 438 (1991) and W. Chen, H. Ohmed, J. Vac. Sci. Technol., B 11, 2519 (1993)).
These types of lithographic processes suffer from several limitations which can become extremely constraining in the fabrication of sub-100 nm devices. These limitations include undesirable proximity effects in the resist and resolution limits imposed by the size of the polymer molecules. Proximity effects are produced when the exposed patterns are situated within the range of backscattered electrons. These electrons are primary electrons which collide with the substrate with a great angle to escape from the surface with a high energy in an area which may be considerably larger than the electron beam diameter. These high energy electrons expose the resist in an undesirable region. Current research efforts in lithography techniques include several resistless processes for defining patterns (see for example D. Wang, P. C. Hoyle, J. R. A. Cleaver, G. A. Porkolab, N. C. MacDonald, J. Vac. Sci. Technol., B 13, 1984 (1995) for electron beams; and H. Sugimura and N. Nakagiri, J. Vac. Sci. Technol., B 13, 1933 (1995) for a scanning probe technique).
The formation of a silicide layer is usually carried out by annealing samples of thin metal layers on silicon substrates in a conventional furnace with a controlled atmosphere of N.sub.2 -H.sub.2. This annealing technique requires several minutes to convert the metal film into silicide (see C. A. Chang, J. Appl. Phys., 58, 3258 (1985); C. A. Chang and A. Segmuller, J. Appl. Phys., 61, 201 (1987); C. A Chang and W. K. Chu, Appl. Phys. Lett., 37, 3258 (1980); and C. A. Chang and J. M. Poate, Appl. Phys. Let., 36, 417 (1980)).
New techniques involving Rapid Thermal Annealing (RTA) improve the process of the formation of silicide. RTA silicide films are significantly better than those formed by conventional annealing (C. A Dimitriadis, Appl. Phys. Lett., 56, 143 (1990)), due to a shorter processing time (A. Torres, S. Kolodinski, R. A. Donaton, K. Roussel and H. Bender, SPIE, 2554, 185 (1995)).
More recently, several techniques of formation of silicide have been developed. These processes involve heating of metal-silicon interfaces using photons, electrons and ion beams (J. M. Poate and J. W. Mayer, Laser Annealing of Semiconductor, Academic Press, New York, 1982; J. Narayan, W. L. Brown and R. A. Lemons, Laser-Solids Interactions and Transient Processing of Materials, North-Holland, N.Y., 1983; and E. D'Anna, G. Leggieri and A. Luches, Thin Solids Films, 129, 93 (1985)). All these processes are based on the concept of forming silicide with localized heating near the surface. However, none of these techniques are intended as lithography processes or for the fabrication of masks for lithography.
Ultra-violet (UV) lithography is the technique used in large scale production of devices and circuits. However, the wavelength of ultra-violet light represents a physical limit to the resolution that can be achieved using this technique. Alternative techniques are being evaluated to replace UV lithography for industrial production of electronic devices and circuits. One of these techniques is X-ray lithography, since X-rays have a much smaller wavelength than UV. In this technique, regions of a mask placed between the X-ray source and the sample are covered by a layer of heavy atoms (such as Ta, W and tantalum suicides) which absorbs X-rays (J. Canning, Journal of Vacuum Science and Technology, B15, 2109 (1997)). Such masks are fabricated using electron-beam lithography. A major challenge in the fabrication of these masks is to use electron-beam lithography to form structures (etch masks) that have both a high resolution and an excellent resistance to chemicals employed to remove the absorbent layer of heavy atoms in unprotected regions. Conventional techniques using polymeric resists as etch masks either do not provide a sufficient resolution or do not provide a sufficient resistance to chemical etching (J. P. Silverman, Journal of Vacuum Science and Technology, B15, 2117 (1997)).